Engine misfires or skips a beat every 20 seconds or so

One annoying thing about the 1994 XR650L that I recently purchased is that at idle speed it will "miss" about every 20 seconds.  I don't know if it's just missing a single spark or more, but it only lasts a fraction of a second.  However, if the idle is too low it's enough for the engine to die because of it.  I haven't really noticed this behavior at higher engine speeds, and other than that  it runs just fine under power.

 

Things I've checked so far:

  • Valve adjustments - one exhaust was a little bit tight, but all are in spec now and no change in behavior after the adjustment
  • ECU - I swapped in a fairly new one from my other bike with no change
  • Carburetion/Jetting - The bike is a lot happier after rejetting, but it didn't change this behavior.  I've verified that there are no air leaks between the carburetor and engine.

Things I expect to check soon:

  • Spark plug - I'll just slap a new one in there to see if anything changes.
  • Coil - I have a new one that I've never used.  I could swap that and see if anything changes.

I can't think of much else to check, but would welcome any suggestions.

If the idle is low you sometimes can hear this,,but not constantly...change the plug Scott ...all but one person i know off changed their coil and did nothing,,coil failure is rare on these bikes...

 

As a side note,,,i decided to try an iridium NGK plug in my bike.......and it works great,,londer lasting to ..

 

B

I put a new spark plug in there to see what would happen.  It is a serious pain to get the old plug out and a new one in there, at least without taking the tank off first, which is how I did it.

 

So the new plug had a slightly smaller gap than the old one, but looked close enough.  I didn't have anything besides the original tools with me, so nothing to measure a gap with.  And I don't want to take that plug back out again to measure.

 

I rode about two miles without any misfires and was thinking that maybe it's fixed now.  But after that it had a couple of small misfires at idle.  It seems to affect the engine speed less than before, so I'll have to put a lot more miles on the bike and see how much of a change the plug has made.

Spray carb cleaner arround the carb and boots too Scott and also intake..see if you might have a small vac leak..

 

B

One of my old XR600 twin carbs used to do that sort of thing,,idling then a sort of miss and it carried on idling. Almost like a slight electrical interference thing..Never figured it out,,that particular bike was just a lemon full stop..Glad to get rid of it

One of my old XR600 twin carbs used to do that sort of thing,,idling then a sort of miss and it carried on idling. Almost like a slight electrical interference thing..Never figured it out,,that particular bike was just a lemon full stop..Glad to get rid of it

Gee, that makes me feel better. :banghead:

 

I'm willing to accept that maybe that's just how this bike is.  I need to go find some dirt to ride it in.

I put a new spark plug in there to see what would happen. It is a serious pain to get the old plug out and a new one in there, at least without taking the tank off first, which is how I did it.

So the new plug had a slightly smaller gap than the old one, but looked close enough. I didn't have anything besides the original tools with me, so nothing to measure a gap with. And I don't want to take that plug back out again to measure.

I rode about two miles without any misfires and was thinking that maybe it's fixed now. But after that it had a couple of small misfires at idle. It seems to affect the engine speed less than before, so I'll have to put a lot more miles on the bike and see how much of a change the plug has made.

I cut the old tank brackets off with my dremel..makes plug removal a breeze. Apparently you still have the orig....tank.

Does it do it hot or cold? try 1/2 choke & see if it gets worst or gone. If choke makes a difference, it would indicate fuel too rich or too lean. Is idle too slow? can auto decompression kick in a little? (just a shot in the dark.)

Aftermarket tank.  I don't know what brand.  Photo is below.

 

It's worse when cold, mainly because it's more likely to kill the engine.  It still happens when hot, though.  The bike was way lean and it did it.  Then I rejetted and now it's on the rich side everywhere and it still does it.

 

Tank2_zpsf545f7c5.jpg

I have a XR4 does the same thing. I've been searching for solutions,and have come across your post(glad I'm not the only one). Valve clearance and mixture are my next targets. I'll let you know how it goes.

I've ruled out both valve clearance and mixture for my bike.

Try a bigger slowjet or enrichen the idle mixture by adjusting the mixture screw

Try a bigger slowjet or enrichen the idle mixture by adjusting the mixture screw

I had the same behavior with lean jetting and rich jetting.  In other words, jetting changes made no difference at all for this particular issue.

Then....swap ign coils and the CDI Scott........if no joy then only thing i can see left is the PG......if fuel didn`t affect it,,it would point towards fire,,,me thinks..

B

Then....swap ign coils and the CDI Scott........if no joy then only thing i can see left is the PG......if fuel didn`t affect it,,it would point towards fire,,,me thinks.

Alreadys swapped the CDI with no change.

 

I was going to swap the coil, but somebody (I thought it was you) indicated that coils never go bad, or something.

 

I'll try that, since I have a brand new, never been used, coil that I can fairly easily swap.

I probably said that......i`ve only known 1 person that a coil actually made a difference,,,,,you could be number 2,,,lol.......i seriously doubt it is the coil Scott,,so stick it in and you get to call me a liar..lol......

 

Could be that stupid resistor in the cap all corroded to crap too...

Hey,,your CDI works really good,did a 200k run,,works sweet..someday when i get off my rear i`ll send it,,maybe soon,who knows.. :p

 B

Edited by brianhare

I pulled the resistor out, checked continuity of the wire and had none. Turns out the spring had some corrosion from the soak jumping as did the end if the lead that contacts the spring. I scraped the contact points of the spring and the end of the wire where it seats and now have continuity and low impedance. I replaced the resistor with a piece is stainless. No change to the miss at idle. But yesterday, a week after the removal of the resistor, i rode for about an hour with out a miss. Today however, not so lucky. The mystery continues

This is an issue that plagued me also.  I tried a larger pilot jet and that was the only thing that seemed to help.  If your situation was the same, it would kind of cough a little bit and then get back to idle.  the jetting helped, but its still there unless the idle is wound out.  

 

my honest feeling is that its a carb problem.  Mine is on a 91 XR6R and i think it is just worn out.  I am replacing the carb with the FCR mod that is a buzz on the forum now.  if it helps, i will let you know.

I've solved my problem.

 

I sold the bike.  The new owner is going to take it all apart, including the top end of the engine and make sure everything is perfect, then take it to Baja for a month or two.  It never did misfire while I was showing it to him.

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